It is well known to use various hollow fibers for various applications. For example, hollow fibers are used in carpets, as fill materials for pillows, as insulation materials for blankets and garments, and as membranes for gas separation, blood dialysis, purification of water, and other filtering applications. For membrane applications, the hollow fibers can be bundled together and disposed in a tubular housing to provide a separation device known as a permeator. Ordinarily, the hollow fibers are relatively small, having a diameter on the order of 30 to 1000 micrometers. Accordingly, the apparatus and method for manufacturing hollow fibers must be very precise to be able to control the diameter of the fiber, and the concentricity of the core around the bore.
Numerous spinning assemblies have been devised for the production of hollow fibers. Particularly, devices have been proposed for ensuring uniform supply of the fiber-forming fluid or fluids to the orifices of a spinnerette with the object of producing hollow fibers identical in diameter, composition, and concentricity. These spinnerettes use a means for supplying the bore fluid positioned in the spinning orifice for forming the bore or “hollow” of the hollow fiber. Usually, a tube or needle is used for this purpose and a gaseous or liquid fluid is injected from the tube, thus forming the bore of the fiber as it is being extruded from the spinnerette orifice. For melt spinning, the nascent fiber can be solidified by cooling in a gaseous or liquid cooling fluid. For solution spinning, the nascent fiber can be solidified by evaporation of the solvent or by contacting the fiber with a solvent-extracting liquid that results in coagulation of the polymer solution(s) to form the fiber wall.
In order to maintain the concentricity of fiber diameter and the bore diameter, other spinnerettes have been provided with members for centering the tube and the bore of the spinnerette plate. For example, U.S. Pat. No. 4,493,629 describes a modular spinnerette assembly fitted with multiple screws threaded through the spinnerette plate to center the tube and orifice of the spinnerette. However, these adjusting screws are unreliable and are prone to error when the spinnerette is disassembled, cleaned and then reassembled.
Typically, spinnerettes of this type are made largely by hand, one at a time. As a result, parts made for one spinnerette will not always fit another spinnerette. When parts are not interchangeable, any damage to one part of the spinnerette assembly may render the entire assembly useless. In assembling or cleaning conventional hollow-fiber spinnerettes, it is very easy to slightly bend the fluid-injection tube or needle, such that it is off center of the spinning orifice. When this happens, the spinnerette cannot be used until repaired.
Yet another problem with existing spinnerette designs is the ability to deliver the polymer fluid uniformly around the tube or needle within the spinnerette. U.S. Pat. No. 5,320,512 discloses a spinneret that has a plurality of discrete material passages formed around the needle to deliver the polymer fluid around the needle. This design enables the uniform delivery of the polymer fluid around the tube or needle within the spinnerette. However, in order to attain concentricity and uniformity in the manufacturing process, the polymer fluid from these individual passages must converge and meld together to form a singular annular flow around the tube or needle as the polymer fluid traverses through the main polymer fluid passage. If complete convergence and melding are not attained, seams may develop down the length of the spun fiber at the interfaces where the individual flows did not fully converge and meld together completely.
Thus, there still remains a need to overcome this problem of interface convergence and melding of the fluid polymer in order to be able to produce a concentric spun hollow fiber with minimal seaming potential.